In many on-line data processing systems, devices remote from each other and from the data processor may communicate with each other over a leased or dial line via the data processor and a connected communications control unit. Typically, such a system may include a multiplicity of remote devices connected via leased or dial lines to an IBM 3705 communications controller which is in turn channel connected to an IBM System 370 computer.
The devices may communicate, for example, using Extended Binary Coded Decimal Interchange Code (EBCDIC) and Binary Synchronous Line Control (BSC). The computer will, for example, be provided with a suitable access program such as the IBM Basic Telecommunications Access Method (BTAM). Other devices and access methods may also be used for effecting the desired device to device communication. The example set forth above is typical of such a connection.
The code set provided in EBCDIC is fixed, however, and provides limited text and control codes. In those instances where the devices communicating with each other require commands and/or instructions not provided in the EBCDIC code set, some provision must be made to encode and communicate these commands and/or instructions from one device to the other via the intervening devices and access programs set forth above as examples. In many access methods new codes not provided for in the code set, such as EBCDIC, are stripped or mutilated by the access program. When this takes place, the commands and/or instructions fail to reach or are mutilated before reaching the recipient device. In either event, the received message, including the commands and/or instructions will not be suitable for use at the recipient device.
Several prior solutions to the above problem are available. None of these solutions is fully satisfactory for a variety of reasons. The access method can be modified to accommodate additional codes which will permit encoding of the necessary commands and instructions. This solution is totally unacceptable in view of the time, effort, and cost of modifying an existing access method. Another solution is to utilize a rigidly formatted command instruction and text format. Such a solution incurs a modest additional cost in machine development and manufacturing which is tolerable from this point of view. However, it requires skilled or trained operators. In machines intended for secretarial personnel unskilled in data processing, use of such a rigid format would inhibit productivity and incur substantial training cost and is, therefore, undesirable.